Book excerpt: Mystery of the Church presents a global picture of the main themes of current ecclesiology. First, it deals with the institution of the Church and her essential nature. Subsequently the four hallmarks of the Church are described. Her unity and holiness are guaranteed by the sanctity of Christ her Head despite the sinfulness of her members. The catholicity of the Church is also examined from the perspective of Eastern Christendom. The apostolicity of the Church leads to a description of the Petrine Office. The Church is seen as the instrument of salvation, and her relationship with the State and with science is investigated. Finally the Church is pictured as leading to the Kingdom of God. This is the fifth volume of Fr Paul Haffner's systematic exposition of theology - previously published volumes in this series: Mystery of Creation, The Mystery of Mary, The Mystery of Reason, The Sacramental Mystery. Paul Haffner is a priest and professor of theology at Regina Apostolorum University in Rome, visiting professor at the Pontifical Gregorian University, and adjunct lecturer at Duquesne University Roman Campus.